1. Field
The embodiments of the present invention relates to a method of correcting a flare and a computer program product.
2. Description of the Related Art
A pattern formed by an exposure apparatus used in manufacturing a semiconductor integrated circuit has been miniaturized year by year. Therefore, it is expected that EUV (Extreme Ultra Violet) whose wavelength is shorter than ArF becomes a mainstream after a half-pitch (HP) 30 nm generation as exposure light used by the exposure apparatus. When the wavelength is λ and an optical numerical aperture is NA, resolution is expressed by an expression λ/NA, and a finer pattern can be formed as this value is smaller. Thus, with the EUV, the resolution of an exposure transfer pattern can be increased due to wavelength-shortening, enabling to form a finer pattern than the ArF.
On the other hand, along with this wavelength-shortening, some changes from a conventional technology occur, such as a device configuration and an exposure method. A lens and a mask are cited as major change points. Conventionally, a refractive lens is used as a lens optical system; however, when exposure is performed using the EUV, exposure light does not penetrate through the refractive lens because of a relationship between light absorption and a refractive index, so that the conventional refractive lens cannot be used and a reflective optical system (mirror) needs to be used.
Moreover, as an exposure mask used for pattern transfer, a reflective mask needs to be used instead of a conventional transmissive mask in which a light shielding area is partially provided on a transparent substrate. This reflective mask is configured to include a reflective area for partially reflecting light so that a desired pattern is exposed on a resist and an absorptive area in which exposure light is prevented from reflecting.
Furthermore, in manufacturing the mirror, a mirror surface needs to be polished; however, the mirror cannot be finished to be completely flat, and concaves and convexes (roughness) are formed on the mirror surface. When such a mirror surface roughness is present, the exposure light radiated to the mirror is reflected diffusely on the mirror surface in the pattern exposure. Therefore, scattered light is radiated to an unintended area on a resist surface of a wafer and thus contrast is lowered to blur a pattern image, so that a finished pattern shape cannot be formed as desired. The scattered light exposing to an unintended area is called flare, which is one of the major factors of an accuracy degradation of the pattern shape in the EUV exposure patterning.
In order to solve the degradation of the pattern shape quality due to the effect of the flare, the pattern shape on the mask is deformed to cancel expansion and contraction of the pattern due to the effect of the flare. However, the flare occurs due to the scattered light (scattered reflection light) that is reflected on a peripheral pattern (reflection area) and the actual circuit pattern is complicated and does not have uniformity, so that a pattern correction corresponding to the arrangements of the peripheral patterns is needed at each correction portion.
In an OPC (Optical Proximity Correction), the pattern correction can be made by taking into account the peripheral pattern in the range (few microns: 10−6 m) of an OPE (Optical Proximity Effect). On the other hand, in the case of the flare, for calculating flare intensity with respect to a target pattern, because the flare is attributed to the roughness of the mirror as described above, it is needed to take into account the effect of the peripheral pattern in a long range of at least millimeter (10−3 m) order. This is one of the major characteristics of an EUV flare correction. For correcting the effect in the long range, a simple correction of adding a correction amount that is predetermined with respect to the flare intensity is performed without dynamically performing a simulation, an iteration, or the like, which is a method that can be processed within a realistic time in view of a current computer processing capability.
Moreover, in the inventions described in U.S. Pat. No. 6,625,802 B2 (Intel) Sep. 23, 2003 (Feb. 1, 2002) and U.S. Pat. No. 6,898,781 B2 (Intel) May 24, 2005 (Jul. 30, 2003), an area of a circuit pattern layout is partitioned (called a grid) into predetermined intervals, and a density of the pattern is calculated for each partitioned area. Then, the calculated density and a PSF (Point Spread Function) are convolved to calculate the flare. At this time, a first flare value in the case of coarsely partitioning (coarse grid) an area in a predetermined distance or further and a second flare value in the case of finely partitioning (fine grid) an area up to the predetermined length are calculated. Then, the value obtained by summing the first flare value and the second flare value is determined as the flare value of the whole area. Moreover, there are described a method of finding a portion where the uniformity of the flare value is poor and a method of finding a portion where the uniformity of the flare value is poor and adding a dummy pattern to correct a flare uniformity.
Furthermore, in the invention described in Japanese Translation of PCT International Application No. 2007-524255, calculation of the flare intensity and the flare correction are performed. As the calculation of the flare intensity, first, a circuit pattern layout is partitioned and a brightness value (pattern density) of each partitioned area is calculated. Next, convolution is performed on each area by using the brightness value and a point spread function (for example, Gaussian or fractal function) to obtain the flare intensity. As the flare correction, an edge is biased to perform the flare correction.
Moreover, in the invention described in U.S. Pat. No. 6,815,129 B1 (EUV LLC) Nov. 9, 2004 (Sep. 26, 2000), the PSF is first determined and the PSF and an areal image of a mask pattern are convolved (convolution) to calculate the flare intensity. Next, a lookup table, in which a bias amount with respect to the flare intensity that is obtained by experiment or the like in advance is defined, is used and the bias amount (correction amount) corresponding to the flare intensity is added to the pattern, to perform the pattern correction (flare correction) to cancel the effect of the flare.
The technology described in Japanese Translation of PCT International Application No. 2007-524255 describes that the edge is biased in accordance with the flare intensity, and the technology described in U.S. Pat. No. 6,815,129 B1 (EUV LLC) Nov. 9, 2004 (Sep. 26, 2000) describes that an exposure experiment is performed by using a mask for the experiment or the like and the correction amount is calculated based on the result thereof. If a portion where there is no flare effect is set to a bias 0, a large correction amount needs to be added to the pattern at a portion having a high flare value. If the large correction amount is added to the pattern, an inter-pattern distance or a pattern width is shortened, which makes it difficult to perform a mask manufacturing or a mask inspection. Moreover, difference between a mask shape and a desired shape leads to various adverse effects such as that degradation of a correction accuracy occurs due to a flare value change along with change in the pattern density before and after correction and an accuracy degradation occurs due to a slight error of MEF estimation, and furthermore, the time required for the correction is increased due to increase of correction target areas and the amount of mask writing data is increased.
For avoiding such problems, there is a method of changing a reference flare value for which the correction amount is 0 so that the correction amount when performing the flare correction does not become large by adjusting an exposure dose or the like at the time of the exposure. However, the value to be set as the reference value is determined based on an empirical judgment by a user at the present stage, and a systematic setting method is not present.
In the technologies described in the above four patent documents, a correction reference is not changed and a method of determining the flare value to be the reference is not described. Therefore, with the technologies described in the above four patent documents, a problem arises that the flare correction cannot be performed accurately. Moreover, the layout of today's semiconductor circuits has become more complex along with miniaturization and multiple functions, so that it has become extremely difficult to determine the reference value by a user.